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Day One of Oscar Pistorius Murder Trial: Scream, Gunshots

<p>"I cannot understand how I could clearly hear a woman scream but Mr. Pistorius could not hear it," neighbor testifies on first day of double-amputee sprinter's murder trial.</p>

PRETORIA, South Africa -- One of Oscar Pistorius' neighbors testified that she heard a "blood-curdling" scream on the night the Olympian gunned down his girlfriend.

On the first day of the world-famous sprinter's murder trial, university lecturer Michelle Burger also told a court in Pretoria, South Africa, that she heard "four gunshots."

Pistorius says he killed model Reeva Steenkamp by mistake thinking she was an intruder in his house, but prosecutors believe the athlete shot his girlfriend after a fight. They immediately tried to paint a picture at the trial of a loud argument before the fatal shots.

Burger's testimony contradicts Pistorius' version of events, because the double-amputee said he thought Steenkamp was in bed and did not describe any woman screaming.

Defense lawyer Barry Roux opened his cross-examination by asking Burger if she thought Pistorius was a liar.

"I can only tell the court what I heard that evening," said Burger, who lives 187 meters away from Pistorius' house and had her windows open. "I cannot understand how I could clearly hear a woman scream but Mr. Pistorius could not hear it."

Erin Conway-Smith, a senior correspondent with NBC News' partner GlobalPost, noted that Roux “rather fiercely” cross-examined Burger.

Earlier, Pistorius pleaded not guilty to murder and other charges.

Defense lawyer Kenny Oldwadge laid out Pistorius' legal strategy, reading a statement from Pistorius in which he says the killing was an accident and that there were inconsistencies in the state's case, as well as an attempt to introduce inadmissible character evidence to discredit him. He also claimed the crime scene had been contaminated.

Steenkamp, 29, was a model and budding reality TV show star when she was slain on Feb. 14, 2013.

The victim's mother will give her first live interview of the trial Tuesday on TODAY.

If convicted on the murder charge, Pistorius could be sent to prison for at least 25 years before the chance of parole. South Africa doesn't have the death penalty.

The trial continues.

Aliza Nadi, Ayman Mohyeldin, Rohit Kachroo and Jason Cumming of NBC News and The Associated Press contributed to this report.